I've been into the hobby for about a year or so and there's a few things I wish I new then that I know now.
1. The starter set with the LionChief engine, O-36 curves and the barrel plug power supply shouldn't be the basis of a layout. I didn't realize that there were tons of engines, cars, etc. that can't run on O-36. When I started planning my larger room-sized layout I didn't know this and it has limited me somewhat.
2. Just buy a good sized transformer and start wiring from the start. I tried to expand my layout with additional Fastrak power adapters. They just don't have the amps to run any sort of good sized layout.
3. There are so many helpful YouTube videos from manufacturers, other folks in the hobby, etc. and maybe watch them before trying to come up with a better mouse trap. I had done a lot of planning and layout work without using the references at my disposal. It would have been helpful to learn more before jumping in.
My layout is an ongoing, evolving process and I can correct or modify things to suit my future needs but I sure wish I knew some of this beforehand.
What do you wish you knew then that you know now?